Aqueous baths and solutions containing chelating agents and chelated heavy metals are often used in the electroless deposition of heavy metals, as well as in metal etching. The electroless deposition process is particularly utilized to deposit nickel on steel or a heavy metal such as copper on plastic. The latter process is widely utilized in the printed circuit industry. These and other industries generate effluent solutions, either as a waste rinsewater stream, or a spent bath concentrate, which contains one or more heavy metals complexed with a chelating agent. The presence of the chelating agent, which bonds strongly to the heavy metals, prevents the routine removal of the heavy metals from these solutions. The removal of the heavy metals is desirous either for recovery, or waste treatment purposes. Effluent concentrations of heavy metals from the above-mentioned sources must normally be reduced to levels of less than about 3 mg/L and sometimes to levels of less than 1 mg/L to meet government regulations.
The conventional technique for treatment of heavy metal bearing wastewaters is to raise the pH of the stream with an alkali such as sodium or calcium hydroxide, causing the metal to precipitate from solution as the hydroxide. The effect of the chelating agent is to hold the metal in solution and prevent this precipitation process. If excess chelating agent is present in the solution, as is often the case, precipitation of metals originating from other effluent streams that may be mixed in will also be impeded.
The two principal chelating agents used in electroless copper plating are ethylenediaminetetraacetic acid (EDTA) and N,N,N',N'-tetrakis-(2-hydroxypropyl)-ethylenediamine (Quadrol). The former, in the inventors' experience, presents particularly difficult problems in removing copper from the solutions in which it is contained.
One known method for treatment of EDTA as well as Quadrol bearing wastewaters, is to lower the pH to between 2.7 and 5.0 and add an iron salt, either in the ferric or ferrous form. (J. V. Peck, R. M. Spearot, "Copper Removal from Metal Finishing Waste Solutions", Proc. 70th Annual Technical Conference of American Electroplaters' Society, June 1983, Indianapolis, Ind.) Lowering the pH weakens the copper complex and facilitates the substitution of the copper for iron. It has been stated that a seven times stoichiometric excess of iron is necesary to displace the copper. The pH is then raised to 9.0, using sodium or calcium hydroxide and copper hydroxide precipitates from solution.
Although effective, the above technique is disadvantageous in that a considerable excess of iron is required. In addition, the separation of themetal hydroxide precipitate from the liquid is, in itself, a difficult problem. The resulting precipitate of copper contains considerable water and is highly contaminated with iron, both of which have the result of rendering the copper precipitate of no value and usually of considerable liability.
U.S. Pat. No. 4,303,704 issued Dec. 1, 1981, to C. Courduvelis et al., teaches a method of removal of copper complexed with alkanolamines, ammonia or carboxylic acids or their salts, or nickel complexed with ammonia or carboxylic acids or their salts. This method employs a chelating type ion exchange resin with an iminodiacetic acid funtionality such as Amberlite IRC 718, manufactured by Rohm and Haas Company. The solution containing the complexed heavy metal ions and complexing ions are retained in the resin bed, while the effluent passing out of the resin bed contains the chelating agent, but is substantially free of complexed heavy metal ions. The retained heavy metal ions are eluted from the resin by passage of an aqueous acid solution through the bed, whereby the eluate contains heavy metal ions and is substantially free of chelating agents.
The process described in U.S. Pat. No. 4,303,704 was found by the inventors to be ineffective in removing heavy metals from solutions containing the chelating agent EDTA.